1. Field of the Invention
The present invention relates to a control apparatus, more particularly relates to a control apparatus suppressing natural vibration of a controlled object including an electric motor and a machine driven by this electric motor while controlling the controlled object.
2. Description of the Related Art
In the past, machine tools equipped with electric motors have used control apparatuses for controlling the electric motors. In particular, a control apparatus using an electric motor, that is, a servo motor, for feedback control controls the difference between position, speed, current, and other command values and the actual values to become zero at all times. The machine position, motor speed, etc. are accurately controlled, so this is being widely employed in general CNC machine tools etc.
On the other hand, a controlled object including a servo motor and a machine driven by this servo motor is a physical dynamic system having elasticity, friction, mass, etc. When its rigidity or attenuation characteristic is low etc., sometimes the controlled object will resonate at the natural frequency and vibration having a large amplitude will occur.
Machines driven by servo motors, for example, include end effecters and workpieces held by hands and other end effecters. Further, if vibration occurs due to resonance, problems are liable to arise such as the machining precision of the machine tool falling or the held workpiece detaching from the hand.
Further, the natural frequency of the controlled object is not necessarily always constant and sometimes changes for the following reason. As the reason, first, sometimes the natural frequency changes depending on the machine position. Giving one example, in the axis of linear movement of a machine tool, when the drive source, that is, the servo motor, drives the driven object through a ball-screw mechanism, the spring constant of the ball-screw mechanism changes and the natural frequency changes between the case where the driven object is near the drive source and the case where it is far from the drive source.
Further, sometimes individual differences in machines result in changes in the natural frequency. Giving one example, mass produced machine tools, even when the same type of machine tools, individually differ to the assembly precision, the mass of the driven object, and other machine specifications and therefore differ in natural frequency.
Further, sometimes the natural frequency changes due to aging of the machine. Giving one example, the natural frequency changes due to wear on the sliding parts, the increase of rattling at the joints, etc. Further, sometimes the natural frequency changes depending on the rigidity of the held workpiece.
Further, a control apparatus reducing natural vibration has been proposed. For example, Japanese Patent Publication (A) No. 7-261853 and Japanese Patent Publication (A) No. 2001-293638 propose a control apparatus removing the natural frequency component from a control signal by a filter so as to reduce the vibration due to resonance.
The control apparatus described in Japanese Patent Publication (A) No. 7-261853 and Japanese Patent Publication (A) No. 2001-293638 continuously detects the natural frequency of the controlled object, changes the frequency which the filter removes in accordance with the detected natural frequency, and thereby prevents the controlled object from resonating.
However, with just continuously detecting the position of the controlled object etc. and detecting the natural frequency of the controlled object as in the control apparatus described in Japanese Patent Publication (A) No. 7-261853 or Japanese Patent Publication (A) No. 2001-293638, due to the following reasons, sometimes it is not possible to accurately detect the natural frequency affecting the operation of the controlled object. The reason is first that sometimes the apparatus detects a large amplitude other than natural vibration of the controlled object such as motor cogging, machining reaction force during machining, etc. and mistakenly judges that frequency to be the natural frequency.
Further, when the controlled object has several natural frequencies, first it is preferable to eliminate the natural vibration having the frequency closest to the control band. This is because the vibration far from the frequency of the control band, but largest in amplitude is sometimes detected as the natural frequency to be removed by the filter.
Further, depending on the state of the controlled object, sometimes no resonance occurs due to the natural frequency, so even if trying to detect it, sometimes it is not possible to detect the natural frequency of the controlled object.
Further, in processing for reducing the natural vibration of a controlled object, sometimes the controlled object is made unstable. As one example, when not able to detect the accurate natural frequency, but detecting the mistaken natural frequency and changing the settings of the filter accordingly, sometimes the controlled object becomes unstable. As another example, even if detecting the accurate natural frequency, when the natural frequency is close to the control band, sometimes the filter strips part of the signal of the control band so again the controlled object becomes unstable.
Still further, there is a demand that the control apparatus stably control the control object under conditions of a limited storage capacity of the apparatus and limited time for control processing.
These are issues which should be dealt with after the control apparatus accurately detects the natural frequency of the controlled object.
As explained above, further improvements are hoped for so as to reduce the resonance due to the natural frequency in a control apparatus controlling a controlled object including an electric motor etc.